The invention relates to a flash tube formed of a hollow-cylindrical tube made of hard glass or quartz. Metallic end caps are attached to each of the ends of the tube so as to be gas-tight and which may support electrodes in the tube interior. The tube is filled with gas, preferably inert gas.
A flash tube of this kind is well-known and described in U.S. Pat. No. 4,099,084, incorporated herein by reference, as a pulse discharge lamp. For example, flash equipment currently in demand by consumers should be more compact and, perhaps, already built into cameras. Therefore, the trend for ever smaller flash tubes exists. A considerable contribution along these lines is to no longer use wire ducts through glass as electrode supply lines, but rather to seal the glass or quartz tube of the flash tube by means of a metallic end cap, wherein these metallic end caps form not only the seal but also perform the function of the electrode supply line. In this way, both functions require only a small portion of the overall constructional length of the flash tube. The discharge length of the gas discharge in the interior of the tube, and which is essential for the light yield, is then nearly the same as the external construction length. The entire discharge space is filled with the light-producing plasma.
This compact type of construction also has the advantage of simple production. Inside a large container, many flash tubes can be filled with gas all at once, and the hermetic sealing can take place under the filling pressure universally prevailing.
If, however, it is desired to increase the filling pressure, especially to obtain a higher light yield, then the hermetic sealing presents several difficulties because the gas pressure rises quickly and becomes high during the gas discharge, and glass-metal type connections do not have much stability against tensile stresses. For this reason it is suggested in U.S. Pat. No. 4,099,084 to attach the metallic end caps, not to the ends of the tube, but to melt them onto the inner surfaces of the tube ends. In this way mainly shear stresses, caused by the pressure, occur, which the melting connection can endure better than tensile stresses.
This arrangement, however, has decisive disadvantages. Not only do the coefficients of thermal expansion of metal and glass have to be matched, but as a consequence of that, expensive tungsten has to be used. Another disadvantage is that, for the purpose of melting the quartz or hard glass necessary for the working load, such high temperatures are necessary that it cannot be guaranteed that the geometrical conditions at the tube ends will be maintained. Also, activating substances, which are located in the cathode for reducing the work function, evaporate and can cause a short-circuiting lining on the inner sides of the tube.